SN Curve (Fatigue Curve) – Theories of Elastic Failure – Strength of Materials


Hello Friends here in this video we will see what is the meaning of SN Curve so let us get started SN curve also called as fatigue curve now SN curve it is a graph between stress and number of cycles plotted for fatigue loading so it is a graph between stress and number of cycles and plotted for fatigue loading your fatigue failure is considered it means a material is subjected to alternate tensile and compressive stresses at the same time so the material is subjected to tensile and compressive stresses at the same time that is the meaning of fatigue stresses and the results of the fatigue testing plotted on a graph is called as SN curve so the results of the fatigue testing which is plotted on the graph is called as SN curve and now I will draw that SN curve to explain it in a more effective manner so now here we have a graph in which on y axis we have bending stresses starting from zero then 100 200 300 400 it is in terms of Newton per mm square similarly on x-axis we have number of cycles of failure that is the object is subjected to 10 raised to 5 cycles that is that much amount of time the stresses are acting on that object so here we have number of cycles now I will plot for two different materials the first one is mild steel this line which I have drawn this indicates mild steel which is also a ferrous metal next this constant line is called as fatigue limit then after that we have one non ferrous metal this I can say that it is aluminium which is a non ferrous metal now see in SN curve it is very much understood that for mild steel if we are operating at a higher amount of stress then it will last for less amount of cycles that is the number of cycles of failure are less then if we go on decreasing the stress for mild steel there comes a point where after reaching a certain number of cycles that is which between 10 raise to 6 and 10 raise to 7 after that it is constant that is the fatigue limit is constant it means the material when it has reached a particular state of a stress then it can go for infinite number of cycles so that is for ferrous metals but for non-ferrous metals if it is operating at less number of cycles and less number of stress then if we along with the stress if we go on decreasing the stress but increasing the number of cycles that is this non ferrous metals they can operate at less amount of stress for more number of cycles so I can say that from this figure which I have drawn this is the SN curve and I can say that ferrous metals can operate at high stresses for less number of cycles whereas non-ferrous metals can operate at less amount of stresses for more number of cycles so your ferrous metals they can operate at high stresses for less number of cycles then after certain stage the stress remains constant and then it can operate for infinite cycles whereas for non-ferrous metals they operate at less stress and if we go on decreasing the stress they can operate at a very huge number of cycles so the number of cycles of failure increases for non ferrous metals so in this video we have seen the SN curve

21 Replies to “SN Curve (Fatigue Curve) – Theories of Elastic Failure – Strength of Materials”

  1. SIr you are wrong about the fatigue limit. For Mild steel can withstand indefinite no of cycles at constant stress is Endurance limit and not fatigue limit.

  2. Hello Friends,

    Watch Complete Video Series of Subject Strength of Materials only on Ekeeda Application.

    Use Coupon Code "NEWUSER" and access any one Subject free for 3 days!

    Download Ekeeda Application and take advantage of this offer.

    Android:- https://play.google.com/store/apps/details?id=student.ekeeda.com.ekeeda_student&hl=en

    iOS:- https://itunes.apple.com/tt/app/ekeeda/id1442131224

Leave a Reply

Your email address will not be published. Required fields are marked *